1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for correcting stray light components detected by an image sensor when spectrum measurement is performed.
2. Description of the Related Art
FIG. 1 is a block diagram showing a configuration of a general spectrum measuring apparatus comprising a diffraction grating and an image sensor. Conventionally, in a spectrum measuring apparatus having such a configuration, it has been known that stray light components (hereinafter referred to as ghosts) other than measurement-target light are detected by the image sensor and that ghosts cause negative effects on precision in the measurement. Ghosts are caused by various factors, such as internal reflection in the housing unit of the spectrum measuring apparatus, surface reflection in the optical lens, reflection in the image sensor, and optical characteristics of the diffraction grating. FIGS. 2A and 2B exemplify cases where ghosts are caused by multiple reflection in the image sensor. FIG. 2A exemplifies a case in which, in measurement of single-wavelength light, a ghost is detected in a position shifted from a regular detection position of measurement-target light because of multiple reflection in the image sensor. FIG. 2B exemplifies a case in which, when an actual measurement sample (hereinafter referred to as a patch) is measured, a signal including a ghost superimposed on the measurement-target light is outputted by the image sensor.
In order to reduce such ghosts, various measures have conventionally been taken. In general, the mechanical system and the optical system of a spectrum measuring apparatus are configured in a way that stray light caused in the apparatus does not enter the image sensor. Furthermore, a technique of removing ghosts by signal processing, without changing the mechanical system and the optical system, is also known. For instance, according to the technique disclosed in Japanese Patent Laid-Open No. 11-30552, a sensor output distribution, which is acquired when single-wavelength light enters a spectrum measuring apparatus, is obtained as a ghost distribution of the single-wavelength light and stored as matrix data. Based on the matrix data and a spectrum distribution of measurement-target light that is acquired by actual patch measurement, a ghost distribution of the measurement-target light is estimated by calculation, such as convolution, and ghost correction is performed.
However, according to the ghost correction method disclosed in Japanese Patent Laid-Open No. 11-30552, a large amount of memory is necessary for holding a large amount of matrix data indicative of ghost distributions. Furthermore, each time patch measurement is performed, it is necessary to convolve each pixel output of the image sensor with the aforementioned matrix data, and a considerable amount of calculation ensues, resulting in equivalent processing speed.